The ‘Beat the Boss’ also double as bluetooth headsets and are compatible with smart phones (Picture: eBay)

The phones are the size of a lipstick and are ‘100 per cent plastic’, ensuring they will not get detected by security scanners. Another tiny phone model is called ‘HMP’ and has a voice changing mode and costs £24.90.

Although the phones are not illegal, eBay has strict rules about not selling items which encourage illegal activity.

An eBay spokesman told Metro.co.uk: ‘We work with the police and regulators to ensure that all of our listings comply with the law. We will also remove items that could imply or encourage illegal activity.’

One former prisoner at HMP Birmingham said: ’10 years ago phones used to be like gold dust, only the top top geezers on the wings would have them. Now they are being passed around the cells all the time.

‘One geezer I was padded up with had brought in four of the small mobiles up his arse, he sold them within minutes of getting on the wing.’

Mobile phones have been flown into jails on drones, thrown over the wall or smuggled inside bodies Leeds Prison, Armley, Leeds (Credit: PA)

Last year 20,075 mobile phones and sim cards were confiscated in prisons in 2016, according to the Ministry of Justice.

As well as being smuggled inside prisoners’ bodies the cheapness of the mini-mobiles means they have been flown into jails on drones or simply thrown over walls.

‘Under measures introduced in late 2016, we can now apply for Telecommunications Restriction Orders at court to block specific mobile phones being used in prisons.

A Prison Service spokesman said: ‘Anyone caught trying to bring a mobile phone into prison, or transmit sounds or images from within a prison using a mobile phone can face up to two years in prison.’

‘We are taking unprecedented action to stop the supply of contraband in our prisons and are stepping up measures to find and block mobiles, including a £2million investment in detection equipment and legislation to block phones from being used in prisons.

‘A range of technology has been rolled out to prisons to strengthen searching and security. Every prison in England and Wales has been equipped with portable detection equipment which can be deployed at fixed points such as in reception, and extra signal detectors to use on wings to support searches.’